Don't know what good signs will accomplish without enforcement. The yahoo's who tear up the countryside, don't give a d*mn about anything. The laws are for "other" people... not for them. Signs ain't gonna stop them.
As a side note.... When I use to travel to the area extensively, about 6 or 7 years ago, I would borrow a friend's mountain bike every now and then and bike up the Pacific Crest Trail to the Springs from the Majove Forks Dam. That way I could avoid CastroLand and the troll-toll.
I saw the sign about no vehicles, and interpreted that to be no "motorized" vehicles. I had encountered a few other mountain bikers at the springs before. That's where I got the idea to enjoy the route, and save some time by riding. Of course there were a few "hairy spots" where I would have to dismount and walk. Sandy places made it tough to ride in a couple of spots as well.
One time, arriving back at the spilway in the afternoon, there were some volunteers cleaning up the place, along with the "Tree Cops" (USFS). I got my hands slapped by a Forrest Service ranger for being on the trail with a "vehicle". The rule is NO vehicles of any type, motorized or not. I played stupid an mentioned that I thought it meant motorcycles and ATVs.
"Yes the sign is a bit ambigious." she said, "But don't do it again." No citation issued... but that was the last time I road a bike to the springs.